This invention provides a process for the reduction of water and carboxylic acid content of an alcohol composition. Also provided is a process for the preparation of a fuel ethanol composition from an ethanol composition comprising water and acetic acid.
Alcohols and other oxygenate compounds are being increasingly used in gasoline compositions for many reasons. Ethanol in particular is being increasingly used in gasoline, and in many jurisdictions the regular grade gasoline available through retail channels contains ethanol.
Alcohols may be obtained through a variety of routes including: synthetic preparation, for example alcohols may be derived from synthesis gas, hydrogenation of carboxylic acids or esters, or by the hydration of alkenes; extraction from natural sources, such as natural fats and oils; and preparation by the fermentation of biomass.
In recent years, there has been considerable interest in the preparation of alcohols by fermentation processes; in particular, since ethanol may be used as a biofuel component in gasoline, there has been particular interest in the preparation of ethanol by the fermentation of biomass. The term “biomass” as used herein refers to any source of organic material from biological origin. Examples of fermentation processes include the direct fermentation of biomass, such as sources of a carbohydrate, to alcohol(s) as well as the fermentation of derivatives of biomass to alcohols. For instance, bioethanol may be obtained by fermentation of feedstocks derived from sugar cane, such as sugar cane molasses and sugar cane juice; sugar beet, such as sugar beet molasses and sugar beet juice; cereal crops, such as corn or wheat, and cereal crop derived feedstocks, such as corn syrup; and lignocellulosic materials, such as fast growing grasses or “energy grasses”.
Alcohols may also be derived from a fermentation process performed on a feed stream comprising carbon monoxide and hydrogen, such as synthesis gas; such processes are referenced and described in WO 2012/062633 A1.
Alcohols may also be prepared via the hydrogenation of carboxylic acids and/or esters. For example WO 2009/063176 A1 discloses a process for the conversion of ethanoic acid into ethanol characterised by the following steps:    1. introducing ethanoic acid and H2 into a primary hydrogenation unit in the presence of a precious metal-based catalyst to produce ethanol and ethyl ethanoate,    2. introducing ethyl ethanoate, from step 1, together with H2, into a secondary hydrogenation unit in the presence of a copper-based catalyst to produce ethanol, and    3. recovering ethanol from step 2.
WO 2010/067079 A1 discloses a process for the preparation of alcohol(s) from alkyl ester(s) wherein hydrogen, carbon monoxide and at least one alkyl ester are brought into contact with a hydrogenation catalyst comprising copper and manganese in a reaction zone to produce at least one alcohol, wherein the molar ratio of hydrogen to carbon monoxide in the reaction zone is in the range of from 100:1 to 1:10.
WO 2009/063173 A1 discloses a process for the production of ethanol from ethanoic acid and H2, characterised by the following steps:    1) introducing ethanoic acid, together with methanol and/or ethanol into an esterification reactor to produce methyl ethanoate and/or ethyl ethanoate,    2) introducing ethanoate from step 1, together with H2, into a hydrogenation unit to produce a stream comprising ethanol, unreacted ethanoate and optionally methanol,    3) separating the resulting stream, from step 2, into unreacted ethanoate and ethanol and optionally methanol,    4) optionally reintroducing ethanoate, from step 3, into the esterification reactor of step 1,    5) using at least a part of the methanol and/or the ethanol of step 3, as the methanol and/or ethanol feed of the esterification reactor of step 1, and    6) recovering ethanol, from step 3.
Due to the requirements in the properties of gasoline, and in order to meet various gasoline specifications around the world, any alcohols or other oxygenate compounds that are to be used in gasoline compositions must be compatible with the base gasoline that it is blended with and must not introduce contaminants that would cause the thus formed gasoline to fail to meet the required properties or specifications. As such, it is necessary to control the levels of certain contaminants in alcohols and other oxygenates that are to be used in gasoline.
Due to the nature of alcohols, in particular the lower alcohols (having from one to four carbon atoms) and especially ethanol, water is often present in significant amounts as it tends to be miscible with lower alcohols and can frequently be difficult to remove, for example due to azeotropic behaviour of water and ethanol mixtures. Additionally, certain methods for the preparation of alcohols either use organic acids during the synthesis process or may lead to the production of trace amounts of organic acids as by-products. The presence of water and acids in gasoline is very strictly limited, and, consequently, the amount of water and acid in alcohol compositions that are to be blended in to gasoline would also be required to be limited.
The use of various desiccants for the drying of solvents, including alcohol compositions, is known in the art, including the use of certain molecular sieves. The use of molecular sieve 3A for the drying of ethanol is known in the art.
The removal of acetic acid from fuel ethanol using ion-exchange resins has been reported in ‘Removal of Acetic Acid from Fuel Ethanol Using Ion-Exchange Resin’ by Huisheng Lv, Yanpeng Sun, Mihua Zhang, Zhonfeng Geng and Miaomiao Ren in Energy Fuels, 26, 7299 (2012). In this article, ethanol compositions containing acetic acid were treated using basic ion-exchange resins in order to reduce the acidity of the composition.
Carboxylic acid may also be removed from alcohol compositions by distillation, for example acetic acid can be removed from ethanol in this way. However, heavier alcohols, which may also be present, and which are permissible impurities in fuel ethanol products, complicate the separation and, in general, any alcohol compositions produced by distillation will need to be dried. Additionally, distillation requires relatively large amounts of energy, leading to significantly increased costs. It would therefore be advantageous to limit the amount of distillation required when separating acids from alcohol compositions.